For a receiver having two frequency mixers, rejection of the image frequency is improved with an oscillator having two outputs in quadrature. The oscillator can also output signals in phase and in phase quadrature from digital receivers. The oscillator can be formed to cause a receiver to have a zero intermediate frequency. To be efficient, these receivers must have signals output by the oscillator having a phase shift of precisely 90.degree.. Furthermore, the spectral purity of these oscillators must be very good, i.e., very low phase noise. This is obtained by a high quality coefficient of the resonant circuit.
There are several approaches for obtaining signals in phase quadrature. Two RC circuits may be used. One of the circuits shifts the phase of the signal output from an oscillator by +45 degrees, and the other circuit shifts the phase of this signal by -45 degrees. The disadvantage of this approach is that, although the phase shift between the two channels is actually 90 degrees within a wide frequency range, this is not true for the amplitude of the signals which must be adjusted as a function of the operating frequency, or variations related to the values of the R and C components.
A double frequency oscillator with two outputs having a phase shift of 180 degrees may also be used when followed by a frequency divider of 2 of the "Johnson counter" type supplying two output signals with a phase shift of 90 degrees. The disadvantage of this approach is that a double frequency oscillator has to be made, and this can be very difficult to operate at high frequencies. This oscillator must have a very precise duty cycle equal to 0.5.
An oscillator with four identical looped back cells can also be used, with each cell introducing a phase shift of 90 degrees. Alternatively, two differential cells may be used to obtain a wired phase shift of 180 degrees. The precision of the phase shift between two cells in this type of oscillator is dependent only on the matching of the cells. The oscillation frequency is the frequency at which the phase shift in an open loop is 0 or 360 degrees. Since the oscillator is formed using four identical cells, the phase shift between each cell is 360/4=90 degrees.
FIG. 1 shows one of the cells in an oscillator of this type according to prior art. This low pass cell is used in its differential form as disclosed in the article "1 GHz Quadrature Sinusoidal Oscillator" by Duncan et al., IEEE CICC 95. The basic cell includes a transistor Q' installed in a common emitter configuration. The collector of the transistor Q' is connected to the DC power supply voltage V.sub.cc through a resistance R'.
The need to obtain low phase noise requires the use of reactive components L and C with the highest possible overvoltage coefficient Q since RC oscillators are too noisy. The basic cell shown in FIG. 1 has a gain greater than 1 and a phase shift of 90 degrees, while having a maximum overvoltage coefficient Q.
The inductance L and the capacitance C connected in series are also connected in parallel to the collector resistance R'. The resistance R represents inductance losses L. The input E to the cell is at the base of the transistor Q'. The output S from the cell forms the common point between the inductance L and the capacitance C.
The phase shift of the cell shown in FIG. 1 at the resonant frequency, i.e., the resonant angular frequency .omega..sub.0, is 90 degrees and its quality coefficient under load is Q.sub.c, such that: ##EQU1## The above equation assumes that the transistor and capacitance are accurate.
The only way to increase the quality coefficient under load Q.sub.c for a given inductance is to reduce R'. The gain G of the cell is given by the relation: ##EQU2## The variable g.sub.m is the transconductance of the transistor.
The gain G must be greater than one to enable operation of the oscillator that includes four identical cells in cascade. The output S from one cell is connected to the input E of the next cell, and the output of the fourth cell is connected to the input of the first cell in the cascade. For example, the orders of magnitude of the value of the components may be as follows. If the inductance L is equal to 5 nH, a value of the quality coefficient of the inductance Q.sub..ltoreq. =L.omega..sub.0 /R=5 is at a frequency of 2 GHz, and tolerating a degradation in the quality coefficient under load of 10% (Q.sub.c =0.9 Q.sub.=), then the result is: EQU R'=1.26 .OMEGA.
Hence, for a gain G&gt;1, a value of the input resistance in common base rib corresponding to 1/g.sub.m is equal to 5.7 .OMEGA.. This corresponds to a value of the collector current I.sub.c equal to 4.4 mA.
This relatively high value of the collector current is undesirable. Furthermore, this type of cell requires at least one follower emitter type stage so that the input impedance of the real transistor does not degrade the quality coefficient Q.sub.c. Furthermore, the value obtained for the resistance R' (1.26 .OMEGA.) is not very realistic when formed in an integrated circuit. This is because the value is close to the values of the parasite resistances of the circuit.